LTP-2557JD 5x7 Dot Matrix LED Display Datasheet - 2.0 Inch Height - AlInGaP Red - 2.6V Forward Voltage - English Technical Document

1. Product Overview

The LTP-2557JD is a single-plane, 5x7 dot matrix LED display module designed for character and symbol presentation. Its primary function is to provide a clear, reliable visual output in various electronic applications requiring alphanumeric or simple graphical information.

1.1 Core Advantages and Target Market

This device offers several key advantages that make it suitable for industrial, commercial, and instrumentation applications. Its low power requirement is a significant benefit for battery-operated or energy-conscious designs. The solid-state construction ensures high reliability and long operational life, as there are no moving parts or filaments to fail. The wide viewing angle provided by the single-plane design allows for clear visibility from various positions, which is crucial for user interfaces and status indicators. The device is categorized for luminous intensity, providing consistency in brightness across production batches. Its compatibility with standard character codes (ASCII and EBCDIC) and ability to be stacked horizontally make it versatile for creating multi-character displays or simple graphics. The target market includes point-of-sale terminals, industrial control panels, test and measurement equipment, medical devices, and any application requiring a robust, simple character display.

2. Technical Specifications Deep Dive

The following sections provide a detailed, objective analysis of the device's key technical parameters as defined in the datasheet.

2.1 Photometric and Optical Characteristics

The display utilizes AlInGaP (Aluminum Indium Gallium Phosphide) high-efficiency red LED chips. This semiconductor material is known for its high luminous efficacy and good performance in the red to amber spectrum. The chips are fabricated on a non-transparent GaAs (Gallium Arsenide) substrate. The package features a gray face with white dots, which enhances contrast and readability.

• Average Luminous Intensity (I_V): Ranges from a minimum of 1300 µcd to a typical 3000 µcd when driven at a peak current (I_p) of 32mA with a 1/16 duty cycle. This parameter defines the perceived brightness of the activated dots.
• Peak Emission Wavelength (λ_p): Typically 656 nm. This is the wavelength at which the optical power output is maximum.
• Dominant Wavelength (λ_d): Typically 640 nm. This is the single wavelength that best describes the color perceived by the human eye, which is a saturated red.
• Spectral Line Half-Width (Δλ): Typically 22 nm. This indicates the spectral purity or bandwidth of the emitted light; a smaller value indicates a more monochromatic output.
• Luminous Intensity Matching Ratio (I_V-m): Maximum of 2:1. This specifies the maximum allowable ratio between the brightest and dimmest dot in the array, ensuring uniform appearance.

2.2 Electrical Parameters

The electrical characteristics define the operating boundaries and conditions for the device.

• Forward Voltage per Dot (V_F): Typically 2.6V, with a maximum of 2.6V at a forward current (I_F) of 20mA. This is the voltage drop across an LED when it is conducting.
• Reverse Current per Dot (I_R): Maximum of 100 µA at a reverse voltage (V_R) of 5V. This is the small leakage current that flows when the LED is reverse-biased.
• Average Forward Current per Dot: 13 mA maximum at 25°C. This is the recommended continuous DC current for reliable operation.
• Peak Forward Current per Dot: 90 mA maximum. This is the absolute maximum instantaneous current, typically relevant for pulsed operation.

2.3 Absolute Maximum Ratings and Thermal Considerations

These ratings define the stress limits beyond which permanent damage may occur. Operation outside these limits is not advised.

• Average Power Dissipation per Dot: 33 mW maximum.
• Operating Temperature Range: -35°C to +85°C. The device is designed to function within this ambient temperature range.
• Storage Temperature Range: -35°C to +85°C.
• Current Derating: The average forward current must be derated linearly from 13 mA at 25°C by 0.17 mA/°C as ambient temperature increases. This is critical for thermal management and preventing overheating.
• Solder Temperature: The device can withstand 260°C for 3 seconds at a point 1/16 inch (approximately 1.6mm) below the seating plane during soldering.

3. Mechanical and Packaging Information

3.1 Physical Dimensions

The device has a matrix height of 2.0 inches (50.80 mm). The package dimensions are provided in the datasheet with all measurements in millimeters. Tolerances are typically ±0.25 mm unless otherwise specified. The exact outline, pin spacing, and overall footprint are critical for PCB (Printed Circuit Board) layout and mechanical integration.

3.2 Pin Connection and Internal Circuit

The display has a 14-pin configuration. The pinout is as follows: Pin 1: Anode Row 5, Pin 2: Anode Row 7, Pin 3: Cathode Column 2, Pin 4: Cathode Column 3, Pin 5: Anode Row 4, Pin 6: Cathode Column 5, Pin 7: Anode Row 6, Pin 8: Anode Row 3, Pin 9: Anode Row 1, Pin 10: Cathode Column 4, Pin 11: Cathode Column 3 (Note: Duplicate function, likely a datasheet annotation consideration), Pin 12: Anode Row 4 (Duplicate), Pin 13: Cathode Column 1, Pin 14: Anode Row 2.

The internal circuit diagram shows a standard common-cathode matrix configuration. The columns are connected to cathodes, and the rows are connected to anodes. This structure allows multiplexing, where a single dot (the intersection of a powered row and a grounded column) is illuminated at any instant. By scanning through rows and columns rapidly, the persistence of vision creates the illusion of a stable character.

4. Application Guidelines and Design Considerations

4.1 Driving the Display

To operate the 5x7 matrix, a multiplexing driver circuit is required. This typically involves a microcontroller or dedicated display driver IC. The driver must sequentially activate each row (anode) while providing the appropriate column (cathode) data for that row. The peak current per dot (I_p) of 32mA mentioned in the test condition for luminous intensity is achieved through pulsed operation at a low duty cycle (1/16). The average current per dot must be kept within the 13 mA rating. For example, driving with a 1/8 duty cycle would require the peak pulse current to be approximately 104 mA to achieve an average of 13 mA, which exceeds the 90 mA peak rating. Therefore, careful calculation of duty cycle and peak current is essential. A series current-limiting resistor is typically required for each column or row line to set the current accurately.

4.2 Thermal Management and Soldering

Adherence to the absolute maximum ratings is paramount. The current derating curve must be followed if the device operates in elevated ambient temperatures. During PCB assembly, the specified soldering profile (260°C for 3 seconds) should not be exceeded to avoid damaging the plastic package or the internal wire bonds. Proper PCB layout with adequate copper area can help dissipate heat, especially if multiple dots are illuminated simultaneously for extended periods.

4.3 Stacking for Multi-Character Displays

The datasheet mentions the device is stackable horizontally. This implies that multiple units can be placed side-by-side to form longer messages. In practice, this requires careful PCB design to align the modules and a driver circuit capable of addressing the increased number of rows and columns (e.g., for two modules, you would still have 7 rows but 10 columns). The driver software must manage the expanded display buffer accordingly.

5. Performance Curve Analysis

The datasheet references typical electrical/optical characteristic curves. While the specific graphs are not provided in the text, standard curves for such devices would typically include:

• Forward Current vs. Forward Voltage (I_F-V_F Curve): Shows the exponential relationship, crucial for designing the driving circuitry and selecting the appropriate current-limiting resistor value.
• Luminous Intensity vs. Forward Current (I_V-I_F Curve): Demonstrates how light output increases with current, typically in a near-linear relationship within the operating range before efficiency droop occurs.
• Luminous Intensity vs. Ambient Temperature: Illustrates the decrease in light output as junction temperature rises, highlighting the importance of thermal management.
• Spectral Distribution: A plot of relative intensity versus wavelength, showing the peak at ~656 nm and the spectral width.

6. Technical Comparison and Differentiation

Compared to older technologies like GaAsP (Gallium Arsenide Phosphide) red LEDs, the AlInGaP technology used in the LTP-2557JD offers significantly higher luminous efficiency, resulting in brighter output for the same input current. The gray face/white dot package provides better contrast than all-red or all-clear packages, especially in high-ambient-light conditions. The 2.0-inch character height is a standard size for medium-range readability, larger than many 0.56-inch or 1-inch modules used in compact devices, making it suitable for applications where the display needs to be read from a distance of several feet.

7. Frequently Asked Questions (Based on Technical Parameters)

Q: Can I drive this display with a constant DC current without multiplexing?
A: Technically, you could power one dot continuously, but to display a full character, multiplexing is necessary due to the matrix architecture. Driving all 35 dots simultaneously at their average current would require a very high total current and power dissipation, which is impractical and likely exceeds package limits.

Q: What is the difference between peak wavelength (656 nm) and dominant wavelength (640 nm)?
A> Peak wavelength is the physical peak of the emitted spectrum. Dominant wavelength is the perceived color point on the CIE chromaticity diagram. The difference is due to the shape of the emission spectrum and the non-linear sensitivity of the human eye (photopic response). The dominant wavelength is more relevant for describing the color seen by a user.

Q: How do I calculate the required series resistor?
A> You need the supply voltage (V_CC), the forward voltage of the LED (V_F, use 2.6V), and the desired forward current (I_F). For multiplexing, use the peak current (I_p) corresponding to your duty cycle to achieve the desired average current. The resistor value R = (V_CC - V_F) / I_p. Ensure the resistor's power rating is sufficient for the pulsed power.

8. Operating Principle Introduction

The device operates on the principle of electroluminescence in a semiconductor p-n junction. When a forward voltage exceeding the diode's threshold is applied, electrons and holes recombine in the active region (the AlInGaP layer), releasing energy in the form of photons (light). The specific composition of the AlInGaP alloy determines the bandgap energy and thus the wavelength (color) of the emitted light. The matrix arrangement is achieved by fabricating multiple individual LED chips and connecting their anodes and cathodes in a grid pattern, allowing control of each intersection (dot) via external electronics.

9. Packaging and Ordering Information

The datasheet specifies the part number as LTP-2557JD. The \"JD\" suffix may indicate specific binning for luminous intensity or other parameters. For precise ordering, the full part number from the manufacturer's system should be used. Standard packaging for such components is typically tape-and-reel for automated assembly or trays/bags for manual prototyping.